c++ - Help understanding boost::bind placeholder arguments -

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i reading stackoverflow post regarding sorting vector of pairs second element of pair. obvious answer create predicate, 1 answer used boost caught eye.

std::sort(a.begin(), a.end(),    boost::bind(&std::pair<int, int>::second, _1) <   boost::bind(&std::pair<int, int>::second, _2));

i've been trying figure out how boost::bind works, or @ least how use it, can't figure out purpose of placeholder arguments _1 , _2 are, , boost documentation doesn't sink in @ all.

could explain specific usage of boost::bind?

p.s. original question: how sort vector of pairs based on second element of pair?

this expression:

boost::bind(&std::pair<int, int>::second, _1) < boost::bind(&std::pair<int, int>::second, _2)

namely, use of < operator defines functor between 2 other functors, both of defined bind.

the functor expected sort needs have operator() looks this:

bool operator()(const t& arg1, const t& arg2);

when you're creating functor using boost's < name holders _1 , _2 correspond arg1 , arg2 of functor you're creating.

the bind call create functor calls ::second of arg1 , arg2

with luck, introduction of lambdas in c++0x make expressions obsolete.


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